The present invention generally relates to digital information processing and particularly relates to document authentication and steganography with digital watermarking and techniques.
There is considerable interest today in authenticating documents and digital watermarking techniques, and various systems and methods have been developed that hide information within document contents with digital watermark. Most of the efforts relating to data hiding, however, have been directed toward non-binary document contents like images.
With gray scale and color images (referred to as images hereafter), the capacity for data hiding is greatly increased due to the non-binary nature of the document contents. For example, each pixel of an image is generally represented in digital form by a number of many bits in length. The increased length of the number is necessary to represent the many hues in color images, and also necessary to represent the many shades in grey scale images. Thus, least significant bit modulation with non-binary data in digital form can accomplish hiding of data that only appears as an imperceptible shift in hue or shading of pixels in the displayed or printed document. This technique, however, is not generally applicable to document contents of a binary nature.
The main difficulty related to hiding data in document contents of a binary nature, like graph or text, lies in the binary nature of the data. In other words, these document contents have only one bit per pixel. Thus, modulation of digits of digital forms of the data results in reversal of the data contents wherever the modulation occurs. As a result, a minimal alteration of bits in a binary graph, for example, can result in a substantial change in the appearance and content of the graph. Overall, data is generally difficult to hide in document contents of this type, but methods have been developed for hiding data in text.
Methods for hiding data in text take advantage of the fact that minimal alteration of bits in a binary graph does not as severely alter appearance and content of text as with graph. Thus, altering the length of vertical strokes, and altering word and/or line spacing are used to hide data in text. For example, conventional space-shifting methods and serif-modification methods are proposed in xe2x80x9cElectronic Marking and Identification Techniques to Discourage Document Copyingxe2x80x9d, J. Brassil, S. Low, N. Maxemchuk, and L. O""Gorman, IEEE Infocom 94, and in xe2x80x9cDocument Marking and Identification using Both Line and Word Shiftingxe2x80x9d, S. H. Low, N. F. Maxemchuk, J. T. Brassil, and L. O""Gorman, Infocom ""95, both incorporated herein by reference. These methods cannot be successfully applied to graph, however, because graph often does not possess the same characteristics as text, even on the pixel level. Thus, there remains a need for a solution to the problems relating to hiding data in document contents of a binary nature, and especially for hiding data in graph. Providing such a solution remains the task of the present invention.
In a first aspect, the present invention is a method for hiding data in document contents. The method comprises receiving information relating to the document contents, wherein at least part of the document contents are of a binary nature, converting the data to halftone pixel groups according to predefined binary classifications associated with combinations of halftone pixels, and replacing a binary portion of the document contents with the halftone pixel groups.
In a second aspect, the present invention is a method for extracting data hidden in document contents. The method comprises receiving information relating to the document contents, wherein the document contents include halftone pixel groups, extracting the halftone pixel groups from the document contents, and converting the halftone pixel groups to the data according to predefined binary classifications associated with combinations of halftone pixels.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.